38 FLIGHT International, 2 January 1964
Missiles and Spaceflight
ACCIDENTAL RENDEZVOUS
By J. A. PILKINGTON, BSc
IN their plans for manned lunar landing, both Soviet andAmerican scientists envisage the use of rendezvous in orbit—the joining together of two separate orbiting satellites as they
circle the Earth. The last four Vostoks have shown by virtue of their
orbits that the Russians are already attempting rendezvous, and the
plans for American astronauts to join their Gemini spacecraft to an
orbiting Agena D have received great publicity. Rendezvous is,
of course, difficult enough to achieve between two co-operating
spacecraft; near-rendezvous by two US satellites, however, will be
unintentionally achieved next month. The minimum distance
between them may be about 10km, which is approaching the prox-
imity attained by the Vostoks.
The two satellites in question, known as 1963-03A and 1963-27A,
are part of a USAF project whose purpose is classified. Both are
Agena D second-stage rockets measuring about 5ft in diameter and
30ft long, which were launched by Thor boosters from Vandenberg
AFB, California, on January 16 and June 29, 1963. They may be
part of either the Discoverer or the Samos programme, but have
probably ceased transmitting by now. At 0824hr GMT on October
16, 1963, they had the orbital elements listed in the table below.
Orbital
Satellite
Inclination (deg)Nodal period (min)
Right ascension of ascending node
Change per dayArgument of perigee
Change per dayPerigee height (km)
Apogee height (km)
Elements
1963-03A
81.8894.60
218.12
-1.083167.1
-3.454460
545
1963-27A
82.3094.79
210.22
-1.022353.0
-3.476490
535
From the table of elements, we see that the difference between the
right ascensions defining the orbital planes of the two objects was
7.9° on October 16. This difference is decreasing at a rate of 0.061°
per day and will become zero 129.5 days later—on February 22,
1964. Although the ascending nodes of both satellites will be
coincident on that date, the longitudes on their crossing latitude
50°N will be 0.5° apart because of the difference in inclinations.
Coincidence of the orbital planes at this latitude will occur eight
days later for Northbound and eight days earlier for Southbound
passages. The two points of coincidence will be between 50°S
and 50°N from February 15 to March 2.
Having considered the orbital planes, the position of the satellites
in their orbits is of some interest. The satellites were both at the
same point in their orbits at noon on October 19,1963, but 1963-O3A
was "catching up" 1963-27A at a rate of 2.92min per day. This
means that 27A is lapped by 03A every 32.45 days and four laps will
be completed 129.8 days later—in fact, by February 26, 1964. Both
satellites are affected slightly by atmosphere drag, however, and
these estimates, made some months in advance, may prove slightly
inaccurate. The lapping nevertheless should occur between
February 23 and 29, and will provide an interesting spectacle for
all who care to watch it.
To the observer on the Earth, both satellites appear to flash
regularly, occasionally as bright as stellar magnitude +1.5. The
average brightnesses are :-
1963-03A from +3.2 to +6.2, flashing every 2.1sec
1963-27A from +3.1 to +5.6, flashing every l.lsec.
The fact that 1963-03A flashes more slowly than 27A may well
be the only means of differentiating between the two when they
are seen crossing the sky together. From the position of the
orbital planes next February, the two satellites will be visible from
the British Isles. In the evenings after twilight they will appear to
travel from the southern sky to the northern sky, and in the morn-
ings before sunrise in the opposite direction. The following table
gives the times when the orbital plane will cross the Greenwich
meridian at latitude 50°N:—
Date 1964 Northbound GMT Southbound GMT 1963-27A
Feb 21-2222-23
23-24
24-25
25-26
26-2727-28
28-29
29-1 Mar
19.4519.37
19.29
19.21
19.12
19.0418.56
18.48
18.39
06.2306.15
06.0705.59
05.5005.42
05.34
05.2605.17
precedes 03A b\about: —
^12 mins+9
+6+3
0-3
-6
9-12
Observers should make the above times 4min later for every
degree they are west of Greenwich, to obtain the time the orbit
crosses their longitude. The separation of the satellites may prove
to be slightly different to that suggested in the last column.
The satellites are unlikely to be overhead at the same time as the
orbital plane, and so the observer would have to search for them
as the plane moves westwards across the sky due to the Earth's
rotation. However, individuals wishing to observe this rendezvous
without undertaking a search may refer to a table to appear next
month in an issue of Flight International.
45O 9O 18O 27O
ANGLE ROUND ORBIT
36O
The orbital heights of the satellites 1963-3A and 1963-27A
Finally, let us examine exactly how close the two satellites will
be. Although the orbits of both objects are fairly circular, the
arguments of perigee will differ by about 176° in late February. This
means, that while one is at its apogee, the other is at perigee. They
will only be close at two -points in their orbits when both their
heights are about 508km (see figure). Under the best circumstances
possible, they would be within 9km of each other for only 6min
since, at the two approaches, one satellite would be changing height
at about 2km per minute and the other at about lkm per minute in
the opposite direction. If the arguments of perigee had been almost
equal, the satellites would have remained very close to each other
for several revolutions, the maximum separation in height being
30km at perigee, and tne minimum perhaps only a few kilometres.
So it appears that 1963-03A and 1963-27A will come together only
to travel their separate ways in the years to come. Unless, of course,
their restartable motors are still capable of bursting to life on a
command from the Earth. . ..